Mixed oxide compounds for casting advanced superalloy materials

ABSTRACT

Mixed oxide compounds La 2  O 3  . 11Al 2  O 3 , NdAlO 3  and nonstoichiometric MgAl 2  O 3  are suitable for making cores and molds for casting advanced superalloy materials such as NiTaC-13.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to materials suitable for making cores and molds for the casting and directional solidification of advanced superalloy materials.

2. Description of the Prior Art

Advanced superalloy materials, such as NiTaC-13, and other similar metal eutectic alloys are cast and directionally solidified at temperatures of about 1700° C and above for upwards of 30 hours exposure thereto. Therefore, cores and molds employed therewith must have high temperature strength and nonreactivity with the molten metal. That is, the mold and core material must not dissolve in the cast molten metal nor form an excessively thick interface compound with the molten metal. The cores also must be compatible with the advanced superalloy to prevent hot tearing, or hot cracking, during solidification.

It is therefore an object of this invention to provide new and improved materials for making cores and molds for the casting of advanced superalloy materials.

Another object of this invention is to provide new and improved materials for making cores and molds for the casting of advanced superalloy materials which are also easily removable from castings without detrimentally affecting the metal casting.

Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the teachings of this invention there is provided a ceramic article suitable for use in the casting and directional solidification of advanced superalloy materials such, for example, as NiTaC-13. The article is made of a mixed oxide compound which is one selected from the group consisting of La₂ O₃ . 11Al₂ O₃, NdAlO₃ and the non-stoichiometric MgAl₂ O₃. The article has a density ranging from about 30 percent to 90 percent to provide a porosity content of from about 10 percent by volume to about 70 percent by volume. The porosity, along with at least some of the pores being interconnected permit the use of the materials to be employed for making cores which minimize hot tearing of the cast metal. The mixed oxide compounds are removed from the casting by dissolving in molten salts.

DESCRIPTION OF THE INVENTION

Three materials have been discovered to be suitable for making cores and molds for the casting and directional solidification of advanced superalloy materials such, for example, as NiTaC-13. These materials are La₂ O₃ . 11Al₂ O₃, NdAlO₃ and nonstoichiometric MgAl₂ O₄. By nonstoichiometric MgAl₂ O₄ is meant that MgAl₂ O₄ is in equilibrium with Al₂ O₃.

The La₂ O₃ content in La₂ O₃ . 11Al₂ O₃ is from 8 mole percent to 9 mole percent. The Nd₂ O₃ content in NdAlO₃ is 50 mole percent. The Al₂ O₃ content in MgAl₂ O₄ is from 80 mole percent to about 50 mole percent.

Each of the above material compositions may be prepared by either of three methods. In a first method a mechanical mixture of the proper amounts of each of the two oxides are prepared. A measured amount of the prepared mixture is then pressed into a desired configuration of a predetermined density wherein the porosity content is from about 25 percent by volume to about 70 percent by volume. The pressed shape in then sintered at a temperature of from about 1400° to about 1900° C. A second method is to mechanically mix the proper amounts of the oxides. The mechanical mixture is then calcined at an elevated temperature of from about 600° to about 1700° C and for a predetermined period of time which is dependent upon the calcining temperature. The time varies from about 20 minutes to about 6 hours. After calcining the material is crushed and ground to a desired particle size. This prepared material is then pressed into a desired configuration having a density ranging from 30 percent to 70 percent. The pressed article is then sintered. The porosity content of the sintered article is from about 10 percent by volume to about 70 percent by volume.

A third method of preparing the material compositions is to mechanically mix the proper amounts of the oxides and then fuse-cast them by heating them close to or above their melting temperature. After fuse casting, the mixture will consist essentially of the desired mixed oxide compound. The fused-cast material is then refined into the desired particle size of from about 10 microns to about 150 microns by suitable milling techniques such as hammer-milling, ball-milling, and the like. The desired core configurations are then prepared from this material.

Complicated shapes may be prepared from materials made by any of the above methods by employing a suitable manufacturing technique such as injection molding, transfer molding, and the like.

The range of porosity recited above is to enable the cast metal to shrink during cooling about a core made of one of the materials without the fear of hot tearing, or hot cracking, of the cast metal occurring. At least 10 percent of the pores should be interconnected to aid in leaching of the material from the cast metal.

Samples of each of the materials were exposed to NiTaC-13 metal at 1800° C in a sessile drop test for a period of 1 hour in flowing argon gas. During this period of time it was observed that the wetting angle was rather large. The wetting angle was about 90° for each of the material samples. This large wetting angle is very desirable for metal surface finish when casting metals around ceramic molds and cores.

Upon cooling the surface of the NiTaC-13 metal was examined first. The NiTaC-13 had not reacted with either one of the materials comprising the ceramic samples. Therefore, the surface finish is acceptable for commercial casting production.

It is believed that the three materials La₂ O₃ . llAi₂ O₃, NdAlO₃ and nonstoichiometric MgAl₂ O₃ each have a more negative free energy of formation than Al₂ O₃. Therefore, NiTaC-13, a metal containing carbon is considerably less susceptible to decarburization when cast in molds or at around cores of these three mixed oxide compounds as compared to when NiTaC-13 is cast in Al₂ O₃ molds or around Al₂ O₃ cores.

The removal of these three mixed oxide compounds, when used as core materials, from the inside of the casting is accomplished by use of molten salts. Preferably, the removal is practiced in an inert atmosphere. Suitable molten salts are molten fluoride salts and/or molten chloride salts such as M₃ AlF₆, M₃ AlF₆ + MF, M₃ AlF₆ + M'F₂ and M₃ AlF₆ + MCl wherein M is Li, Na or K and M' is Mg, Ca, Ba or Sr. The temperature of the molten salt for dissolving the core material is of course dependent upon the salt or salt mixture employed and the rate of dissolution of the core material desired. 

We claim as our invention:
 1. A ceramic article suitable for use in the casting and directional solidification of advanced superalloy materials comprisinga mixed oxide compound material La₂ O₃ . 11Al₂ O₃, and a porosity content which is from about 10 percent by volume to about 70 percent by volume.
 2. The ceramic article of claim 1 wherein the La₂ O₃ content is from about 8 mole percent to about 9 mole percent of the mixed oxide compound. 